8.2 Biomineral Contents and Shapes
Examples of calcium-bearing biominerals are found in the echinoderms, a phylum
of marine invertebrates. Biomineral formation has an important role in the
phylum, which includes Echinoids (sea urchins and sand dollars), Asteroids (sea
stars), Ophiuroids (brittle stars), Crinoids (feather stars) and Holothuroids (sea
cucumbers). It supports the construction of the skeleton of the living organisms,
offering support and protection. Echinoderms employ a genetically regulated process to form their calcareous skeleton by the precipitation of magnesian calcite, a
form of calcium carbonate that contains small amounts of magnesium carbonate in
a ratio given by the formula: (Mg x Ca 1-x )CO 3 . In a high Mg/Ca ratio, amorphous
calcium carbonate or aragonitic nucleation is favored over calcite (Raz et al. 2000;
Mann 2001).
The magnesium carbonate content can vary from 2.5% to 39% depending on the
classes and species. In general, the hardness of the biomineral is directly proportional to the concentration of magnesium. Low magnesium biominerals are found in
the softer Ophiuroids and high magnesium biominerals are found in the harder
Asteroids (Dubois and Chen 1989).
The skeletons of all echinoderms are made of microscopic bony plates, also
called ossicles, deposited as a three-dimensional meshwork called a stereom, made
of magnesian calcite and a network of interconnected holes filled with living tissue.
Different types of stereom are indicative of the type of living tissue that penetrates
the plates (e.g., cells, tube feet, others). The skeleton is covered by an epidermis and
contains a network of internal water-filled canals or encloses a coelomic cavity
bathed in coelomic fluid. The structure of the biomineral reflects complexity.
Skeletal plates may remain simple or may fuse to form composite plates. They
can also form tubercles, granules, fixed, or movable spines. Examples of the
diversity of stereoms within the one species, Paracentrotus lividus (P. lividus),
can be observed in Fig. 8.1. Among the classes, the most diverse skeletal
organizations are evident in Echinoids and Holothuroids. In the former, organisms
are completely surrounded by a calcified test embedded in the mesodermal stroma
tissue, with only a thin layer of muscular tissue. On the contrary, Holothuroids
possess an endoskeleton reduced to microscopic ossicles dispersed throughout
the dermis, with a highly muscularized body wall (Smith et al. 2010). In the
last 40 years, advanced high-resolution imaging techniques, such as SEM and
TEM, have helped researchers to provide information about the fine structure and
composition of the stereom, as well as the cellular basis of echinoderm
calcification.
Various studies have been aiming to identify the proteins involved in the mineralization of the adult sea urchin test, teeth, and spines (Berman et al. 1993). The
proteome of the mineralized parts of the adult sea urchin was recently described
using mass spectrometry-based methods. In the Strongylocentotus purpuratus (S.
purpuratus) sea urchin, 138 (Mann et al. 2008a) and 110 (Mann et al. 2008b)
proteins were identified in the tooth and test/spines organic matrix respectively.
228
V. Matranga et al.
Examples of calcium-bearing biominerals are found in the echinoderms, a phylum
of marine invertebrates. Biomineral formation has an important role in the
phylum, which includes Echinoids (sea urchins and sand dollars), Asteroids (sea
stars), Ophiuroids (brittle stars), Crinoids (feather stars) and Holothuroids (sea
cucumbers). It supports the construction of the skeleton of the living organisms,
offering support and protection. Echinoderms employ a genetically regulated process to form their calcareous skeleton by the precipitation of magnesian calcite, a
form of calcium carbonate that contains small amounts of magnesium carbonate in
a ratio given by the formula: (Mg x Ca 1-x )CO 3 . In a high Mg/Ca ratio, amorphous
calcium carbonate or aragonitic nucleation is favored over calcite (Raz et al. 2000;
Mann 2001).
The magnesium carbonate content can vary from 2.5% to 39% depending on the
classes and species. In general, the hardness of the biomineral is directly proportional to the concentration of magnesium. Low magnesium biominerals are found in
the softer Ophiuroids and high magnesium biominerals are found in the harder
Asteroids (Dubois and Chen 1989).
The skeletons of all echinoderms are made of microscopic bony plates, also
called ossicles, deposited as a three-dimensional meshwork called a stereom, made
of magnesian calcite and a network of interconnected holes filled with living tissue.
Different types of stereom are indicative of the type of living tissue that penetrates
the plates (e.g., cells, tube feet, others). The skeleton is covered by an epidermis and
contains a network of internal water-filled canals or encloses a coelomic cavity
bathed in coelomic fluid. The structure of the biomineral reflects complexity.
Skeletal plates may remain simple or may fuse to form composite plates. They
can also form tubercles, granules, fixed, or movable spines. Examples of the
diversity of stereoms within the one species, Paracentrotus lividus (P. lividus),
can be observed in Fig. 8.1. Among the classes, the most diverse skeletal
organizations are evident in Echinoids and Holothuroids. In the former, organisms
are completely surrounded by a calcified test embedded in the mesodermal stroma
tissue, with only a thin layer of muscular tissue. On the contrary, Holothuroids
possess an endoskeleton reduced to microscopic ossicles dispersed throughout
the dermis, with a highly muscularized body wall (Smith et al. 2010). In the
last 40 years, advanced high-resolution imaging techniques, such as SEM and
TEM, have helped researchers to provide information about the fine structure and
composition of the stereom, as well as the cellular basis of echinoderm
calcification.
Various studies have been aiming to identify the proteins involved in the mineralization of the adult sea urchin test, teeth, and spines (Berman et al. 1993). The
proteome of the mineralized parts of the adult sea urchin was recently described
using mass spectrometry-based methods. In the Strongylocentotus purpuratus (S.
purpuratus) sea urchin, 138 (Mann et al. 2008a) and 110 (Mann et al. 2008b)
proteins were identified in the tooth and test/spines organic matrix respectively.
228
V. Matranga et al.